Technical Field
The present invention relates to an isolated signal transmission apparatus for isolatingly transmitting, from a control-side apparatus to a control target apparatus, an electric signal obtained by superimposing a communication signal on a DC (direct-current) current signal and, more particularly, to an isolated signal transmission apparatus for enabling a control target apparatus to operate even during failure of the control side apparatus.
Related Art
In a plant, an operation signal from a host device, such as a distributed control system (DCS), is input to a field device that performs a control operation, such as a valve manipulation. Generally, a DC current signal of 4 to 20 milliamperes (mA) is used as an operation signal. An isolator and a distributer are known as an isolated signal transmission apparatus that isolatingly transmits a DC current signal.
In recent years, what is called smart communication has widely been performed, in which a DC current signal is transmitted by superimposing a digital communication signal thereon. An isolated signal transmission apparatus dealing with the smart communication is enabled to isolatingly transmit a communication signal, in addition to a DC current signal of 4 to 20 mA.
FIG. 10 is a block diagram illustrating an example of application of an isolated signal transmission apparatus that performs isolated transmission of a signal in the smart communication. This figure illustrates an example of an isolated signal transmission apparatus that isolatingly transmits, to a valve positioner serving as a control target apparatus from a DCS 410 serving as a control-side apparatus, an electric signal obtained by superimposing a communication signal on a DC current signal of 4 to 20 mA. An output isolator 500 is used as the isolated signal transmission apparatus. In the output isolator 500, a side, to which the DCS 410 is connected and to which an electric signal is input, is referred to as a system side, while a side, to which the valve positioner 420 is connected and from which an electric signal is output, is referred to as a field side.
The valve positioner 420 also uses the DC current signal of 4 to 20 mA as an operating power source, and manipulates the degree of opening of a valve 430 according to an electric signal isolatingly transmitted by the output isolator 500.
FIG. 11 is a block diagram illustrating a configuration of a related-art output isolator 500. As illustrated in this figure, the output isolator 500 separates a communication signal and a DC current signal from an electric signal input to an input terminal of the system side by a capacitor Ci, and isolatingly transmits the communication signal to the field side by a communication signal isolating circuit 540. An isolation transformer, an isolation amplifier, or the like is used as the communication signal isolating circuit 540. Incidentally, the transmission using the communication signal is performed bi-directionally. Thus, a capacitor Co is also provided in the field side.
The DC current signal of 4 to 20 mA is converted by a direct-current signal input circuit 550 into, e.g., DC (direct-current) voltage signal of 1 to 5 volts (V) that is isolatingly transmitted to the field side by a direct-current signal isolating circuit 520. A photocoupler or the like is used as the direct-current signal isolating circuit 520. Then, the DC voltage signal of 1 to 5 V is converted by a direct-current signal output circuit 530 into a DC current signal of 4 to 20 mA.
The communication signal and the DC current signal of 4 to 20 mA, which are isolatingly transmitted to the field side, are superimposed on each other again and output from an output terminal.